Exhaust emission control apparatus for internal combustion engine

ABSTRACT

One object of the present invention is to provide an exhaust emission control apparatus for an internal combustion engine that is capable of improving the NOx purification performance. 
     An exhaust emission control apparatus being placed in an exhaust path of an internal combustion engine, comprising: an NOx retention material; ozone injection means for injecting ozone into an exhaust gas being positioned upstream from the NOx retention material; an HC adsorbent adsorbing HC contained in the exhaust gas; and a catalyst being placed in a region where desorbed NOx and desorbed HC contact each other. The HC adsorbent is prepared so as to desorb adsorbed HC at the same time as NOx being stored in the NOx retention material is desorbed.

TECHNICAL FIELD

The present invention relates to an exhaust emission control apparatusfor an internal combustion engine.

BACKGROUND ART

It is known that a conventional exhaust emission control apparatusdisclosed, for instance, in JPA-2002-89246 includes an NOxStorage-Reduction catalyst (hereinafter referred to as an “NSRcatalyst”). More specifically, the NSR catalyst comprises a catalystfunction which purifies nitrogen oxide (NOx) and hydrocarbons (HC)contained in the combustion gas exhausted in the internal combustionengine, and a storage function which stores NOx in the catalyst. Beforethe catalyst activates, such as when there is a cold startup of theinternal combustion engine, or while the exhaust gas is lean, and whenthe reducing agent is in short supply, NOx may be emitted into theatmosphere without being purified in the catalyst. NSR catalyst iscapable of storing NOx in the catalyst, and performing a purifyingtreatment by reacting the stored NOx and reducing agents such as HCunder a situation where the catalyst has been activated and rich orstoichiometric driving is performed.

Moreover, during a cold startup operation of the internal combustionengine in which the NSR catalyst does not reach its activationtemperature, the above mentioned storage reaction may not occur with ahigh degree of efficiency because oxidation reaction of NOx is notactivated. In the above described conventional system, therefore, it isdecided to add ozone to exhaust gas. NOx reacts with ozone in thegas-phase. Therefore, the system can effectively oxidize NOx even beforethe catalyst is fully activated so as to increase the NOx storageamount.

Patent Document 1: JP-A-2002-89246

Patent Document 2: JP-A-6-185343

Patent Document 3: JP-A-10-169434

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

NOx stored in the catalyst is decomposed to N₂, H₂O, CO₂, and others byreducing agents in the exhaust gas such as HC or CO. Duringstoichiometric operation, therefore, the reducing agent for reducing thestored NOx may be in short supply, thereby NOx purification may not beperformed effectively.

The present invention has been made to solve the above problem. Oneobject of the present invention is to provide an exhaust emissioncontrol apparatus for an internal combustion engine that is capable ofimproving the NOx purification performance.

Means for Solving the Problem

First aspect of the present invention is an exhaust emission controlapparatus being placed in an exhaust path of an internal combustionengine, comprising:

an NOx retention material;

ozone injection means for injecting ozone into an exhaust gas beingpositioned upstream from the NOx retention material;

an HC adsorbent adsorbing HC contained in the exhaust gas; and

a catalyst being placed in a region where desorbed NOx and desorbed HCcontact each other;

wherein, said HC adsorbent is prepared so as to desorb adsorbed HC atthe same time as NOx being stored in the NOx retention material isdesorbed.

Second aspect of the present invention is the exhaust emission controlapparatus according to the first aspect, wherein the HC adsorbent andthe NOx retention material are integrated with the catalyst.

Third aspect of the present invention is the exhaust emission controlapparatus according to the first aspect, wherein the NOx retentionmaterial is separated from the HC adsorbent,

wherein the HC adsorbent is positioned upstream from the NOx retentionmaterial, and

wherein the ozone injection means injects ozone into the exhaust gasbeing positioned downstream from the NOx retention material.

Fourth aspect of the present invention is the exhaust emission controlapparatus according to the third aspect, wherein the NOx retentionmaterial is separated from the catalyst, and

wherein the NOx retention material is positioned upstream from thecatalyst.

Fifth aspect of the present invention is the exhaust emission controlapparatus according to the third aspect, wherein the NOx retentionmaterial is combined with the catalyst.

Sixth aspect of the present invention is the exhaust emission controlapparatus according to any one of the first to the fifth aspects,further comprising:

control means for controlling the ozone injection means so that a molarquantity of ozone to be injected into the exhaust gas becomes greaterthan a molar quantity of nitrogen monoxide contained in the exhaust gas.

Seventh aspect of the present invention is the exhaust emission controlapparatus according to the sixth aspect, wherein the control meanscontrols the ozone injection means so that the molar quantity of ozoneto be injected into the exhaust gas becomes at least two times the molarquantity of nitrogen monoxide contained in the exhaust gas.

ADVANTAGES OF THE INVENTION

According to the first aspect of the present invention, an HC adsorbentwhich adsorbs HC is placed in an exhaust path of an internal combustionengine. The HC adsorbent is prepared so that the adsorbed HC might bedesorbed when the NOx being stored by the NOx retention material reachesits desorbed temperature. Therefore, the present invention can make theNOx desorbed from the NOx retention material along with the HC desorbedfrom the HC adsorbent to react with the catalyst so as to be purified,thereby preventing the reducing agents from being in short so as toimprove the NOx purification performance.

According to the second aspect of the present invention, it is possibleto make the desorbed HC and the desorbed NOx react in the catalyst witha high degree of efficiency since the HC adsorbent and the NOx retentionmaterial are integrated with the catalyst. Further, it is possible tohold down production costs since the number of parts is reduced by theintegration.

According to the third aspect of the present invention, ozone isintroduced into downstream from the NOx retention material. Therefore,the present invention makes it possible to prevent ozone from reactingwith HC which is adsorbed into the HC adsorbent, thereby improving theoxidation efficiency of NOx.

It is thought that the storage element which is supported by the NOxretention material could be a catalyst poison for a noble metal of thecatalyst. According to the forth aspect of the present invention, sincethe NOx retention material is separated from the catalyst, it ispossible to enhance the activation of the catalyst. Furthermore, sincethe catalyst is positioned downstream from the NOx retention material,it is possible to purify NOx, which has not been stored by the NOxretention material, in the catalyst after a certain level of catalystactivation has been reached. Therefore, this makes it possible toimprove the NOx purification performance.

According to the fifth aspect of the present invention, the NOxretention material is combined with the catalyst. Therefore, this makesit possible to make the desorbed NOx react in the catalyst with a highdegree of efficiency. Furthermore, the number of parts is reduced bycombining them into a single unit. Therefore, this makes it possible tosave on production costs.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the hardware configuration of a firstembodiment of the present invention.

FIG. 2 is a diagram showing an NOx storage reaction.

FIG. 3 is a diagram showing an NOx reduction reaction.

FIG. 4 is a diagram illustrating the hardware configuration of theexperiment of the present evaluation test.

FIG. 5 is a cross-section diagram illustrating the inside of thecatalyst test piece 200 of the present experiment.

FIG. 6 is a graph illustrating the purification efficiencies for a fewkinds of components in the first and the second experiments.

FIG. 7 is a diagram illustrating the hardware configuration of thesecond embodiment of the present invention.

FIG. 8 is a diagram illustrating an internal configuration of an exhaustpurification catalyst 60 which is able to be applied as modifications ofthe exhaust purification catalyst 42.

FIG. 9 is a diagram illustrating the hardware configuration of theexperiment of the present evaluation test.

FIG. 10 is a cross-section diagram illustrating the inside of thecatalyst test pieces 300, 310 of the present experiment.

FIG. 11 is a graph illustrating the purification efficiencies for a fewkinds of components according to the first and the second experiments.

DESCRIPTION OF REFERENCE CHARACTERS

-   -   10 exhaust emission control apparatus    -   12 internal combustion engine (engine)    -   14 exhaust path    -   20 exhaust purification catalyst    -   30 ozone supply device    -   32 air inlet    -   34 ozone injection orifice    -   40 exhaust emission control apparatus    -   42 exhaust purification catalyst    -   44 NSR catalyst    -   46 HC adsorbent    -   50 ECU (Electronic Control Unit)    -   60 exhaust purification catalyst    -   62 HC adsorbent    -   64 NOx retention material    -   66 three-way catalyst    -   100 model gas generator    -   102 gas cylinders    -   110,112 exhaust gas analyzer    -   114 ozone analyzer    -   120 ozone generator    -   122 oxygen cylinder    -   124,128,130 flow rate control unit    -   126 ozone analyzer    -   200 catalyst test piece    -   202 quartz tube    -   204,206 catalyst sample    -   300,310 catalyst test piece    -   302,312 quartz tube    -   304,314,316 catalyst sample

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described withreference to the accompanying drawings. Like elements in the drawingsare designated by the same reference numerals and will not beredundantly described. It should be understood that the presentinvention is not limited to the embodiments described below.

First Embodiment Configuration of First Embodiment

FIG. 1 is a diagram illustrating the hardware configuration of a firstembodiment of the present invention. As shown in FIG. 1, an exhaustemission control apparatus 10 of the present embodiment is installed inan internal combustion engine (hereinafter referred to as an “engine”)12.

An exhaust path 14 is attached to an exhaust side of the engine 12. Anexhaust purification catalyst 20 is installed in the exhaust path 14.The exhaust purification catalyst 20 supports platinum (Pt) that is anoble metal and barium carbonate (BaCO₃) on a ceramic carrier. Ptfunctions as an active site that activates the oxidation reaction of CO,HC, and others or the reduction reaction of NOx. Further, BaCO₃functions as an NOx storage substance which stores NOx in the form ofnitrate salt.

Zeolite based ZSM-5 is supported by a ceramic carrier of the exhaustpurification catalyst 20. ZSM-5 functions as an HC adsorbent thatadsorbs HC. The following explanation assumes that the terms “catalyst”refers to a portion of the carrier that supports Pt. Further, it is alsoassumed that the term “NOx retention material” refers to a portion ofthe carrier that supports BaCO₃. Furthermore, it is also assumed thatthe term “HC adsorbent” refers to a portion of the carrier that supportsZSM-5.

The exhaust emission control apparatus 10 according to the firstembodiment includes an ozone supply device 30. The ozone supply device30 includes a system that generates ozone by performing a creepingdischarge into oxygen taken from an air inlet 32. The configuration,function, and other characteristics of the ozone supply device 30 willnot be described in detail because they are not main parts of thepresent invention and are well-known.

The ozone supply device 30 includes an ozone injection orifice 34 thatinjects ozone toward the exhaust purification catalyst 20. The ozoneinjection orifice 34 is placed so that the injected ozone might beuniformly mixed with the exhaust gas which is streamed into the exhaustpurification catalyst 20.

As shown in FIG. 1, the exhaust emission control apparatus 10 accordingto the present embodiment includes an ECU (Electronic Control Unit) 50.An output section of the ECU 50 is connected to the ozone supply device30 described above. An input section of the ECU 50 is connected tovarious sensors which detect an operating condition and the operatingstatus of the engine 12. In accordance with the various inputinformation, the ECU 50 calculates an injection timing and an injectionquantity for ozone and controls the ozone supply device 30.

Operation of First Embodiment (NOx Storage Operation Using Ozone)

An operation of the present embodiment will now be described withreference to FIGS. 2 and 3. In the exhaust purification catalyst 20, NOxis decomposed to N₂, H₂O, CO₂, and others by reacting with HC or CO. Asa result, the NOx contained in the exhaust gas is purified effectively.However, during a cold startup of the engine 12 in which the exhaustpurification catalyst 20 is not warmed up until its activationtemperature, the NOx contained in the exhaust gas is not purified in theexhaust purification catalyst 20 because it has not activated enough.

In view of the above circumstances, the present embodiment prevents theunpurified NOx from being emitted into the atmosphere by storing theminto the exhaust purification catalyst 20. Ozone is injected into theexhaust gas as a means for storing the NOx in the exhaust purificationcatalyst 20. More specifically, the ozone generated by the ozone supplydevice 30 is injected from the ozone injection orifice 34 toward theexhaust purification catalyst 20.

FIG. 2 is a diagram showing an NOx storage reaction. More specifically,FIG. 2 is an enlarged view showing the exhaust purification catalyst 20.As shown in FIG. 2, NOx is oxidized by being reacted with ozone in thegas-phase before the exhaust purification catalyst 20 is fullyactivated, i.e., the catalyst is at a low temperature. Morespecifically, the following reactions occur.

NO+O₃→NO₂+O₂  (1)

NO₂+O₃→NO₃+O₂  (2)

NO₂+NO₃→N₂O₅  (3)

(NO₂+NO₃←N₂O₅)

The NOx oxidized to NO₂, NO₃, or N₂O₅ is stored in the NOx retentionmaterial in the form of nitrate such as Ba(NO₃)₂. Thereby, it ispossible to store the NOx effectively. Therefore, it is possible toeffectively prevent the NOx from being emitted into the atmosphere, evenwhen the exhaust purification catalyst 20 is not fully activated.

Moreover, according to the gas-phase reaction of ozone as shown in theabove formulas (1) and (2), the region in which NOx is oxidized is notlimited to a region on catalyst. Therefore, for instance, it is possibleto support the NOx storage substance and noble metal separately in theexhaust purification catalyst 20. Therefore, the NOx storage substancewhich could be a catalyst poison for the noble metal can be separatedfrom the noble metal, thus improving the purification efficiency of thecatalyst.

According to the aforementioned NOx storage reaction, the forms of NO₂,NO₃, N₂O₅, or HNO₃ which is generated when the former nitrogen oxidesreact with water can be stored. However, there is a limitation of NOxstorage amount as for the form of NO₂. Therefore, it is possible todramatically increase the storage amount of NOx if the reaction as shownin the above formula (2) occurs actively so as to convert large amountsof NO₂ into NO₃.

However, the activation energy necessary to take place the reaction asshown in the above formula (2) is higher than the energy required in thereaction as shown in the above formula (1). Therefore, the reaction ofthe above formula (1) takes priority over the reaction of the aboveformula (2). Therefore, all reactions occurring are just the reactionsof the above formula (1) so that the final form of oxidized NO becomesNO₂ when the molar quantity of ozone contained in the exhaust gas issmaller than the molar quantity of NO.

In view of the above circumstances, the injection quantity of ozone iscontrolled so that the molar quantity of ozone to be injected into theexhaust gas might be greater than the molar quantity of NO contained inthe exhaust gas. This make it possible to be oxidized from NO₂ to NO₃.Therefore, it is possible to increase the storage amount of NOx.

Further, it is preferred that the injection quantity of ozone iscontrolled so that the molar quantity of ozone to be injected into theexhaust gas might be more than twice the molar quantity of NO containedin the exhaust gas. This makes it possible for most of the NO to beoxidized to NO₃. Therefore, it is possible to increase the storageamount of NOx dramatically.

(Reduction Operation of the Stored Nox)

As described above, according to the present embodiment, the ozone isinjected toward the exhaust purification catalyst 20. This makes itpossible to store the NOx, which is exhausted with a cold startup of theengine 12, effectively. Therefore, it is possible to prevent the NOxfrom being emitted into the atmosphere.

After the exhaust purification catalyst 20 activates, the stored NOx ispurified by reacting with a reducing agent such as HC contained in theexhaust gas. FIG. 3 is a diagram showing an NOx reduction reaction.Specifically, FIG. 3 is an enlarged view showing the exhaustpurification catalyst 20. As shown in FIG. 3, the stored NOx isdecomposed to N₂, H₂O, CO₂, and others by being reduced with a reducingagent such as HC contained in the exhaust gas after the catalystactivates. Therefore, if the NOx reduction reaction could take placewith a high degree of efficiency, this makes it possible to purify theNOx which is stored before the catalyst activates.

However, NOx may be emitted into the atmosphere without being purifiedin the catalyst during a stoichiometric operation of the internalcombustion engine 12 since the reducing agent for purifying the desorbedNOx may be in short supply. In view of the above circumstances, an HCadsorbent is placed into the exhaust purification catalyst 20 in thepresent embodiment. The HC adsorbent is configured by making the carriersupport a zeolite based adsorbent that adsorbs HC at low temperaturesand desorbs the HC at high temperatures. Desorption temperaturecharacteristics of the adsorbed HC vary depending on the substance ofthe adsorbent. According to the present embodiment, the HC adsorbent isprepared so that the desorption start temperature of HC might coincidewith the desorption start temperature of NOx which is stored in theexhaust purification catalyst 20.

The HC contained in the exhaust gas is adsorbed by the HC adsorbentduring a cold startup of the engine 12. The adsorbed HC is desorbed atthe time when desorption of the stored NOx is started after a rise inthe temperature of the exhaust purification catalyst 20. Therefore, itis possible to reliably purify the NOx stored before the catalystactivation by additionally supplying HC that functions as a reducingagent during the desorption period of NOx when reducing agents are aptto be in short supply.

The first embodiment, which has been described above, assumes that ozoneis generated by the ozone supply device 30 and injected toward theexhaust purification catalyst 20 from ozone injection orifice 34.However, the configuration of the ozone supply device 30 is not limitedto the above-described one. That is, an ozone generation device may beinstalled in the exhaust path 14 or the ozone supply device 30 mayinject ozone into the exhaust path 14 upstream from the exhaustpurification catalyst 20, as long as ozone can be added to the exhaustgas introduced into the exhaust purification catalyst 20.

Furthermore, although BaCO₃ is used as an NOx storage substance in theabove described first embodiment, the material is not limited to this.For example, alkali metals such as Na, K, Cs and Rb, alkali earth metalssuch as Ba, Ca and Sr, and rare earth elements such as Y, Ce, La and Prmay be used as needed. Also, the material of the catalyst is not limitedto Pt. For example, noble metals such as Rh and Pd may be used asneeded. Furthermore, the material of the HC adsorbent is not limited toZSM-5. For example, various publicly known HC adsorbent may be used asneeded.

In the first embodiment, which has been described above, the ozonesupply device 30 corresponds to the “ozone supply means” according tothe first aspect of the present invention; and the exhaust purificationcatalyst 20 corresponds to the “catalyst” according to the first aspectof the present invention.

Evaluation Test for First Embodiment [Configuration of Experiment]

Next, evaluation test for confirming the advantage of the inventionshowing the first embodiment will now be described with reference toFIGS. 4 to 7. FIG. 4 is a diagram illustrating the hardwareconfiguration of the experiment of the present evaluation test. As shownin FIG. 4, the present experiment includes a model gas generator 100.The model gas generator 100 is able to mix the gases which are suppliedin a plurality of gas cylinders 102 to create the simulant gas whichrepresents the exhaust gas of the internal combustion engine.

A catalyst test piece 200 is installed a path which is placed downstreamfrom the model gas generator 100. An electric furnace, which controlsthe temperature of the catalyst test piece 200 to a desired temperature,is installed around the catalyst test piece 200. The configuration ofthe catalyst test piece 200 will be described in detail below.

Exhaust gas analyzers 110, 112 and an ozone analyzer 114 are attached apath which is placed downstream from the catalyst test piece 200. Thesimulant gas which is generated by the model gas generator 100 is gonethrough inside of the catalyst test piece 200 and then it is analyzedits gas components by these analyzers.

Further, the experiment shown in FIG. 4 includes a system in order toinject an injection gas containing ozone. More specifically, the presentexperiment includes an oxygen cylinder 122. An ozone generator 120 isconnected downstream from the oxygen cylinder 122 via a flow ratecontrol unit 124. The ozone generator is a device in order to generateozone from oxygen which is supplied in the oxygen cylinder 122. Theozone generator 120 is connected upstream from the catalyst test piece200 via an ozone analyzer 126 and a flow rate control unit 128. Further,the oxygen cylinder 122 is connected upstream from the ozone analyzer126 directly via a flow rate control unit 130 which is placedseparately. According to the aforementioned configuration, it ispossible to control the ozone amount and the oxygen amount contained inthe injection gas separately.

Following measuring instruments are used for the present evaluationtest.

-   -   Ozone generator 120; Iwasaki Electric, OP100W    -   Ozone analyzer 126; Ebara Jitsugyo, EG600    -   Ozone analyzer 114; Ebara Jitsugyo, EG2001B    -   Exhaust gas analyzer 110; Horiba, MEXA9100D (HC/CO/NOx        measurement)    -   Exhaust gas analyzer 112; Horiba, VAI-510 (CO₂ measurement)

Next, a specific configuration of the catalyst test piece 200 will nowbe described in detail. FIG. 5 is a cross-sectional view illustratingthe inside of the catalyst test piece 200 of the present experiment.According to the present evaluation test, a catalyst test piece 200 awas used as the catalyst test piece 200 in a first experiment, and acatalyst test piece 200 b was used as the catalyst test piece 200 in asecond experiment. FIG. 5( a) is a cross-sectional view illustrating theinside of the catalyst test piece 200 a being used in the firstexperiment. As shown in FIG. 5( a), the catalyst test piece 200 aincludes a catalyst sample 204, which comprises an HC adsorbingfunction, an NOx storage function, and a catalyst function, inside of aquartz tube 202. The catalyst sample 204 was created by performing theprocedure described below.

(Creating Procedure of Catalyst Sample 206)

First, γ-Al₂O₃ was dispersed in ion-exchange water. An aqueous solutionof barium acetate was then added in it. The resulting mixture heated toremove water from it, dried at 120 degrees Celsius, and pulverized topowder. The powder was then burned for two hours at 500 degrees Celsius.After burning, the burnt powder was immersed in a solution containingammonium hydrogen carbonate, and then dried at 250 degree Celsius. As aresult of that, barium-supported catalyst was obtained.

Next, the barium-supported catalyst was dispersed in ion-exchange water.An aqueous solution containing dinitro-diammine platinum was then addedto support Pt. The resulting mixture was dried, pulverized, and burnedfor one hour at 450 degrees Celsius. According to this catalyst, thesupported quantity of barium was 0.1 mole per 120 g of γ-Al₂O₃, and thesupported quantity of Pt was 2 g.

H⁺ ion of ZSM-5 type zeolite (Si/Al ratio=40, H-type) was exchanged ionusing silver nitrate, dried, and then burned at 450 degrees Celsius toprepare Ag ion-exchange ZSM-5. The Ag ion-exchange ZSM-5 was mixed withγ-Al₂O₃ being supported Pt and Ba to obtain a catalyst. The mixtureratio (ZSM-5: Al₂O₃) was 5:12.

A 30 mm diameter, 50 mm long, 4 mil/400 cpsi cordierite honeycomb wascoated with the catalyst being prepared earlier and burned for one hourat 450 degrees Celsius. The coating amount was such that Al₂O₃ wascoated at a rate of 120 g/L. Therefore, the Ag ion-exchange ZSM-5 wouldbe coated at a rate of 50 g/L.

Meanwhile, FIG. 5( b) is a cross-sectional view illustrating the insideof the catalyst test piece 200 b being used in the second experiment. Asshown in FIG. 5( b), the catalyst test piece 200 b includes a catalystsample 206, which comprises an NOx storage function and a catalystfunction, inside a quartz tube 202. The catalyst sample 206 was createdby performing the procedure described below.

(Creating Procedure of Catalyst Sample 204)

First, γ-Al₂O₃ was dispersed in ion-exchange water. An aqueous solutionof barium acetate was then added in it. The resulting mixture heated toremove water from it, dried at 120 degrees Celsius, and pulverized topowder. The powder was then burned for two hours at 500 degrees Celsius.After firing, the burnt powder was immersed in a solution containingammonium hydrogen carbonate, and then dried at 250 degree Celsius. As aresult of that, barium-supported catalyst was obtained.

Next, the barium-supported catalyst was dispersed in ion-exchange water.An aqueous solution containing dinitro-diammine platinum was then addedto support Pt. The resulting mixture was dried, pulverized, and burnedfor one hour at 450 degrees Celsius. According to this catalyst, thesupported quantity of barium was 0.1 mole per 120 g of γ-Al₂O₃, and thesupported quantity of Pt was 2 g.

A 30 mm diameter, 50 mm long, 4 mil/400 cpsi cordierite honeycomb wascoated with the catalyst being prepared earlier and burned for one hourat 450 degrees Celsius. The coating amount was such that Al₂O₃ wascoated at a rate of 120 g/L.

[Experiment Conditions]

Following experiment conditions are used for the present evaluationtest.

Temperature: control a temperature in the range 30 degrees Celsius-500degrees Celsius

Temperature rise rate: 10 degrees Celsius/min.

Simulant gas compositions:

-   -   stoichiometric compositions    -   C₃H₆ (1000 ppm=3000 ppmC)    -   CO (6500 ppm)    -   NO (1500 ppm)    -   O₂ (7000 ppm)    -   CO₂ (10%)    -   H₂0 (3%)    -   Remainder N₂

Simulant gas flow rate: 30 L/Min.

Injection gas compositions:

-   -   O3 (30000 ppm)    -   Remainder N₂

Injection gas flow rate: 6 L/min.

[Test Method]

The electrical furnace being placed around the catalyst test piece 200was controlled so that its temperature might be raised fromlow-temperature side. The injection gas was injected when thetemperature was in the range of 30 degrees Celsius-300 degrees Celsius.Moreover, when the temperature was 300 degrees or higher, only simulantgas was injected without supplying the injection gas. The experimentswas performed both the first experiment being used the catalyst sample204 and the second experiment being used the catalyst sample 206. Anexhaust gas purification efficiency was calculated as a percentage bysubtracting the amount of NOx being emitted into downstream from thecatalyst test piece 200 (hereinafter referred to as an “NOx outletflow”) from the amount of NOx being introduced into the catalyst testpiece 200 (hereinafter referred to as an “NOx inlet flow”) within thetest time and then being divided by the NOx inlet flow within the testtime.

NOx inlet flow=NOx concentration of simulant gas×simulant gas flowrate×test time  (4)

NOx outlet flow=NOx concentration of downstream from the catalyst200×gas flow rate (simulant gas and injection gas)×test time  (5)

purification efficiency=(NOx inlet flow−NOx outet flow)/NOx inletflow×100  (6)

[Results of Test]

FIG. 6 is a graph illustrating the purification efficiencies for a fewkinds of components in the first and the second experiments. As shown inFIG. 6, it indicates that the catalyst test piece being used for thefirst experiment exhibited higher purification efficiencies for NOx, HC,and CO. The aforementioned results indicate an advantage by adding theHC adsorbent to the NSR catalyst.

Second Embodiment Characteristic Configuration of Second Embodiment

Next, the second Embodiment of the present invention will now bedescribed with reference to FIG. 7. FIG. 7 is a diagram illustrating thehardware configuration of the second embodiment of the presentinvention. Elements in the exhaust emission control apparatus 40 shownin FIG. 7 that are common with those in the exhaust emission controlapparatus 10 shown in FIG. 1 are designated by the same referencenumerals, and redundant description thereof will be omitted.

As shown in FIG. 7, an exhaust purification apparatus 40 of the presentembodiment includes an exhaust purification catalyst 42 which isinstalled in the exhaust path 14. An NSR catalyst 44, in which platinum(Pt) as noble metal and barium carbonate (BaCO₃) are supported by theceramic carrier, is placed in the exhaust purification catalyst 42. Ptfunctions as an active site which activates the oxidation reaction ofCO, HC and others or the reduction reaction of NOx. Further, BaCO₃functions as an NOx storage substance which stores NOx in the form ofnitrate.

An HC adsorbent 46, in which zeolite based ZSM-5 is supported by aceramic carrier, is placed in the exhaust purification catalyst 42upstream from the NSR catalyst 44. The ZSM-5 functions as an HCadsorbent which adsorbs HC.

The exhaust emission control apparatus 40 according to the secondembodiment includes an ozone supply device 30. The ozone supply device30 includes an ozone injection orifice 34. The ozone injection orifice34 is placed upstream from the NSR catalyst 44 and downstream from theHC adsorbent 46, and injecting ozone toward the NSR catalyst 44.

Characteristic Operation of Second Embodiment

In the exhaust emission control apparatus 10 according to the abovedescribed first embodiment, it is assumed that the exhaust purificationcatalyst 20 in which the HC adsorbent, the NOx storage substance and thenoble metal are supported on the common carrier is provided so that theNOx exhausted during a cold startup of the engine 12 is effectivelystored in order to be prevented from being emitted into the atmosphere.However, according to the aforementioned configurations, the ozone isalso injected into the HC adsorbent which is placed inside of theexhaust purification catalyst 20. Therefore, the injected ozone mayreact not only with the NOx but also with the HC adsorbed in the HCadsorbent, thereby decreasing an oxidized rate of NOx.

In view of the above circumstances, the second embodiment uses theexhaust purification catalyst 42, in which the HC adsorbent is separatedfrom the NOx retention material. In the exhaust purification catalyst42, the introduced ozone is effectively prevented from reacting with theHC adsorbed in the HC adsorbent 46 since the HC adsorbent 46 ispositioned upstream from the ozone injection orifice as shown in FIG. 7.Therefore, the HC adsorbed in the HC adsorbent 46 can be certainlysupplied to the NSR catalyst 44, thereby efficiently improving the NOxpurification capability.

Further, according to the exhaust purification catalyst 42, since theNSR catalyst 44 is positioned downstream from the ozone injectionorifice 34, all of the injected ozone is used for reacting with NOx.Therefore, it is possible to improve the oxidation rate of NOx andincrease the storage amount of NOx.

Although the NSR catalyst 44 in which the noble metal Pt and BaCO₃functioning as an NOx storage substance are supported by the commonceramic carrier is used in the second embodiment, the configuration ofthe NSR catalyst 44 is not limited to this. For example, according tothe gas-phase reaction of the above formulas (1) and (2), the region foroxidizing the NOx is not limited to a region on catalyst. Thus, itbecomes possible, for instance, to support the NOx storage substance andnoble metal separately in the NSR catalyst 44. More concretely, the NOxstorage substance and the noble metal may be supported separately, forinstance, at the upstream side and the downstream side of the carrier,respectively.

FIG. 8 is a diagram illustrating an internal configuration of an exhaustpurification catalyst 60 which is able to be applied as modifications ofthe exhaust purification catalyst 42. As shown in FIG. 8, an HCadsorbent 62 is installed inside of an exhaust purification catalyst 60.Further, an NOx retention material 64 is installed downstream from theHC adsorbent 62. Furthermore, a three-way catalyst 66 is installeddownstream from the NOx retention material. Furthermore, the ozoneinjection orifice 34 is installed upstream from the NOx retentionmaterial 64 and downstream from the HC adsorbent 62. The ozone injectionorifice 34 injects the ozone toward the NOx retention material 64.

According to the aforementioned configurations, it is possible toimprove the purification performance of the catalyst since the three-waycatalyst in which the noble metal is supported and the NOx retentionmaterial 64 in which the NOx storage substance that can be a catalystpoison is supported are installed separately. Further, the NOx that isnot stored by the NOx retention material 64 can be purified in thethree-way catalyst 66 after the catalyst is activated for some extentsince the three-way catalyst 66 is positioned downstream from the NOxretention material 64, whereby the NOx purification performance isimproved.

Further, the interior configuration of the NSR catalyst 44 is notlimited to the configuration in which the NOx storage substance and thenoble metal are supported separately at the upstream side and thedownstream side of the carrier; a configuration in which an upper layerside and a lower layer side are used for supporting them separately canbe also used as the interior configuration. It should be noted that, ina case where the layered configuration is employed, it is preferablethat the noble metal is arranged as the upper layer on the NOx storagesubstance so that the NOx desorbed from the NOx storage substance isreduced by the noble metal.

Although the exhaust purification catalyst 42 that has the HC adsorbent46 and the NSR catalyst 44 therein is used in the above described secondembodiment, the configuration of the catalyst is not limited to this.For example, the HC adsorbent 46 and the NSR catalyst 44 may beinstalled into the exhaust path 14 respectively without combining theminto a single unit. Further, in this case, the various kinds ofconfigurations can be taken as the ozone supply device 30. For instance,in order to add ozone to the exhaust gas which is flown into the NSRcatalyst 44, an ozone generation device may be installed in the exhaustpath 14 or the ozone may be injected from the ozone supply device 30into a certain part of the exhaust path 14 which is positioneddownstream from the HC adsorbent 46 and upstream from the NSR catalyst44.

Further, although BaCO₃ is used as the NOx storage substance in thesecond embodiment, the material of the NOx storage substance is notlimited to this. For example, alkali metals, such as Na, K, Cs and Rb,alkali earth metals such as Ba, Ca and Sr, and rare earth elements suchas Y, Ce, La and Pr may be used as needed. Further, the material of thecatalyst is not limited to Pt. For example, noble metals such as Rh andPd may be used as needed. Furthermore, the material of the HC adsorbentis not limited to ZSM-5. For example, various publicly known substancesfor adsorbing HC may be used as needed.

It should be noted that in the above described second embodiment, theozone supply device 30 corresponds to the “ozone supply means” accordingto the first aspect of the present invention; and the exhaustpurification catalyst 42 corresponds to the “catalyst” according to thefirst aspect of the present invention.

Evaluation Test for Second Embodiment [Configuration of Experiment]

Next, evaluation test for confirming the advantage of the inventionshowing the second embodiment will now be described with reference toFIGS. 9 to 11. FIG. 9 is a diagram illustrating the hardwareconfiguration of the experiment of the present evaluation test. Likeelements between the experiment shown in FIG. 9 and the experiment shownin FIG. 4 are designated by the same reference numerals and will not beredundantly described.

As shown in FIG. 9, a catalyst test piece 300 is installed downstreamfrom the model gas generator 100. Further, a catalyst test piece 310 isinstalled downstream from the catalyst test piece 300. The ozonegenerator 120 is installed upstream from the catalyst test piece 300 anddownstream from the catalyst test piece 310 via the ozone analyzer 126and the flow rate control unit 128.

Next, concrete configurations of the catalyst test pieces 300, 310 willnow be described in detail. FIG. 10 is a cross-sectional viewillustrating the inside of the catalyst test pieces 300, 310 of thepresent experiment. According to the present evaluation test, thecatalyst test pieces 300 a and 310 b were used in a first experiment,and the catalyst test pieces 300 b and 310 b were used in a secondexperiment. FIG. 10( a) is cross-section diagrams illustrating theinside of the catalyst test pieces 300 a and 310 a being used in thefirst experiment. As shown in FIG. 9( a), the catalyst test piece 300 aincludes a catalyst sample 304 comprising an HC adsorbing functioninside of a quartz tube 302. Further, the catalyst test piece 310 aincludes a catalyst sample 314 comprising an HC adsorbing functioninside of a quartz tube 312. The catalyst samples 304 and 314 werecreated by performing the procedure described below.

(Creating Procedure of Catalyst Sample 306)

First, H⁺ ion of ZSM-5 type zeolite (Si/Al ratio=40, H-type) wasexchanged ion using silver nitrate, dried, and then burned at 450degrees Celsius to prepare Ag ion-exchange ZSM-5. Next, A 30 mmdiameter, 50 mm long, 4 mil/400 cpsi cordierite honeycomb was coatedwith the catalyst being prepared earlier and burned for one hour at 450degrees Celsius. The coating amount was 50 g/L.

(Creating Procedure of Catalyst Sample 314)

First, γ-Al₂O₃ was dispersed in ion-exchange water. An aqueous solutionof barium acetate was then added in it. The resulting mixture heated toremove water from it, dried at 120 degrees Celsius, and pulverized topowder. The powder was then burned for two hours at 500 degrees Celsius.After burning, the burnt powder was immersed in a solution containingammonium hydrogen carbonate, and then dried at 250 degree Celsius. As aresult of that, barium-supported catalyst was obtained.

Next, the barium-supported catalyst was dispersed in ion-exchange water.An aqueous solution containing dinitro-diammine platinum was then addedto support Pt. The resulting mixture was dried, pulverized, and burnedfor one hour at 450 degrees Celsius. According to this catalyst, thesupported quantity of barium was 0.1 mole per 120 g of γ-Al₂O₃, and thesupported quantity of Pt was 2 g.

A 30 mm diameter, 50 mm long, 4 mil/400 cpsi cordierite honeycomb wascoated with the catalyst being prepared earlier and burned for one hourat 450 degrees Celsius. The coating amount of Al₂O₃ was 120 g/L.

[Experiment Conditions]

Following experiment conditions are used for the present evaluationtest.

Temperature: control a temperature in the range 30 degrees Celsius-500degrees Celsius

Temperature rise rate: 10 degrees Celsius/min.

Simulant gas compositions:

-   -   stoichiometric compositions    -   C₃H₆ (1000 ppm=3000 ppmC)    -   CO (6500 ppm)    -   NO (1500 ppm)    -   O₂ (7000 ppm)    -   CO₂ (10%)    -   H₂0 (3%)    -   Remainder N₂

Simulant gas flow rate: 30 L/Min.

Injection gas compositions:

-   -   O3 (30000 ppm)    -   Remainder N₂

Injection gas flow rate: 6 L/min.

[Test Method]

The electrical furnaces which are placed around the catalyst test pieces300 and 310 were controlled so that its temperature might be raised fromlow-temperature side. The injection gas was injected when thetemperature was in the range of 30 degrees Celsius-300 degrees Celsius.Moreover, when the temperature was 300 degrees or higher, only simulantgas was injected without supplying the injection gas. The experimentswere performed both the first experiment being used the catalyst samples304, 314 and the second experiment being used the catalyst sample 306.An exhaust gas purification efficiency was calculated by using aboveformula (5).

[Results of Test]

FIG. 11 is a graph illustrating the purification efficiencies for a fewkinds of components according to the first and the second experiments.As shown in FIG. 11, it indicates that the catalyst test piece beingused for the first experiment exhibited higher purification efficienciesfor NOx, HC, and CO. The aforementioned results indicate an advantage byinstalling the HC adsorbent upstream from ozone injection orifice.

1. An exhaust emission control apparatus being placed in an exhaust pathof an internal combustion engine, comprising: an NOx retention material;ozone injection means for injecting ozone into an exhaust gas beingpositioned upstream from the NOx retention material; an HC adsorbentadsorbing HC contained in the exhaust gas; and a catalyst being placedin a region where desorbed NOx and desorbed HC contact each other;wherein, said HC adsorbent is prepared so as to desorb adsorbed HC atthe same time as NOx being stored in the NOx retention material isdesorbed.
 2. The exhaust emission control apparatus according to claim1, wherein the HC adsorbent and the NOx retention material areintegrated with the catalyst.
 3. The exhaust emission control apparatusaccording to claim 1, wherein the NOx retention material is separatedfrom the HC adsorbent, wherein the HC adsorbent is positioned upstreamfrom the NOx retention material, and wherein the ozone injection meansinjects ozone into the exhaust gas being positioned downstream from theNOx retention material.
 4. The exhaust emission control apparatusaccording to claim 3, wherein the NOx retention material is separatedfrom the catalyst, and wherein the NOx retention material is positionedupstream from the catalyst.
 5. The exhaust emission control apparatusaccording to claim 3, wherein the NOx retention material is combinedwith the catalyst.
 6. The exhaust emission control apparatus accordingto claim 1, further comprising: control means for controlling the ozoneinjection means so that a molar quantity of ozone to be injected intothe exhaust gas becomes greater than a molar quantity of nitrogenmonoxide contained in the exhaust gas.
 7. The exhaust emission controlapparatus according to claim 6, wherein the control means controls theozone injection means so that the molar quantity of ozone to be injectedinto the exhaust gas becomes at least two times the molar quantity ofnitrogen monoxide contained in the exhaust gas.
 8. An exhaust emissioncontrol apparatus being placed in an exhaust path of an internalcombustion engine, comprising: an NOx retention material; ozoneinjection device for injecting ozone into an exhaust gas beingpositioned upstream from the NOx retention material; an HC adsorbentadsorbing HC contained in the exhaust gas; and a catalyst being placedin a region where desorbed NOx and desorbed HC contact each other;wherein, said HC adsorbent is prepared so as to desorb adsorbed HC atthe same time as NOx being stored in the NOx retention material isdesorbed.